Two genes interact to produce various phenotypic ratios among F₂ progeny of a dihybrid cross. Design a different pathway explaining each of the F₂ ratios below, using hypothetical genes R and T and assuming that the dominant allele at each locus catalyzes a different reaction or performs an action leading to pigment production. The recessive allele at each locus is null (loss-of-function). Begin each pathway with a colorless precursor that produces a white or albino phenotype if it is unmodified. The ratios are for F₂ progeny produced by crossing wild-type F₁ organisms with the genotype RrTt.

15/16 black : 1/16 white

Key Concepts

Dihybrid Cross

A dihybrid cross involves two traits, each represented by two alleles, allowing the study of inheritance patterns for two genes simultaneously. In this case, the genes R and T are being analyzed for their interactions in producing phenotypes. The typical phenotypic ratio for a dihybrid cross of two heterozygous parents (RrTt x RrTt) is 9:3:3:1, but interactions between genes can alter these ratios, as seen in the question.

Epistasis

Epistasis occurs when the expression of one gene is affected by another gene, leading to modified phenotypic ratios. In the context of the question, the dominant alleles of genes R and T may interact in such a way that they both contribute to pigment production, while the recessive alleles lead to a loss of function. This interaction can result in a 15:1 ratio, where the presence of at least one dominant allele from either gene results in a black phenotype.

Phenotypic Ratio

Phenotypic ratio refers to the relative frequency of different phenotypes in the offspring resulting from a genetic cross. In this scenario, the 15:1 ratio indicates that out of 16 offspring, 15 exhibit the black phenotype due to the presence of at least one dominant allele, while 1 offspring is white due to the absence of both dominant alleles. Understanding how these ratios arise from genetic interactions is crucial for predicting outcomes in genetic crosses.